Are they eating, drinking and playing?
Do they chirp and chatter?
Do you have lots of toys for them and rotate them so the budgie's minds are stimulated?
Do you spend time interacting with your budgies and give them praise and attention regularly?
Do they grind their beaks at night or make little chirping noises as they settle in for a nice nap?
Are their eyes bright and clear?
Do your provide baths for them?
Do you give them a balanced nutritious diet including pellets, seed mix, vegetables and fruits?

If you can answer yes to the questions above, then I'd say your budgie's are happy and comfortable.

Well if they are quiet and not moving around they are probably scared. If they are chattering, beak grinding, and climbing around they are more comfortable with their surroundings. It's hard to tell with birds sometimes whether their comfortable or scared.

Mine ate some of the fruit I put in there...I need to put some toys back in and give them more food variety. I was trying to neutralize their territory to eliminate the territorialness but i think i have let it go on too long!!!!!!!!!!!!!!!!!!!!!!
They hardly ever chatter, if at all, but they chirp!!!!!!!!!!

when i let my birds out into the out side flight ..you sould here them ....there like a load od school kids ...on break time ...lol...they love it ...i open them at 630am ...and shut them back up ...as its getting dark ......the iner flight isnt very big ...but they have enough room so they can all perch without fighting ....and settle down for the night ...iv only got 19 ...budgies ...i like them to have lots of room ...

It can be hard to tell for different birds!
Most of my budgies make it pretty clear when they are happy- chattering, playing, beak grinding etc.
We often worry that Lily is happy, she is a perch potato and dosnt do a lot, except with Stevie (preening and flirting). She is a very quiet bird. But she may have been quiet and inactive all day, yet start headbobbing so much all of a sudden (means happy/excited) so I guess being quiet is just her personality

The secret to a happy budgie (I think) is being busy all day, and having variety in their life
My budgies love going outside, getting fresh veggies, background noise, being chatted to, coming out to fly, chewing up destructible toys...
But being active and chirpy, those are generally good indicators

Mine ate some of the fruit I put in there...I need to put some toys back in and give them more food variety. I was trying to neutralize their territory to eliminate the territorialness but i think i have let it go on too long!!!!!!!!!!!!!!!!!!!!!!
They hardly ever chatter, if at all, but they chirp!!!!!!!!!!

Try putting music on, all day every day. Mine looove it, and they're chirping and singing more than they ever had. They are really happy.